Nonaqueous electrolytes are used for high energy density secondary batteries and, for example, many nonaqueous electrolyte secondary batteries that are charged and discharged by moving lithium ions between a positive electrode and a negative electrode are in use.
These nonaqueous electrolyte secondary batteries are used as the sources of power for various portable devices, but there is currently a strong demand for nonaqueous secondary batteries capable of even higher energy density in view of the increased power consumption of increasingly multifunctional portable devices.
In recent years, lithium manganese oxides have received attention as inexpensive materials for a positive electrode active material because they use manganese, for which there are more abundant resources than for lithium cobalt oxide, which has conventionally been used as a positive electrode active material.
In nonaqueous electrolyte secondary batteries that use lithium-manganese oxides, which have rhombic crystal systems (belonging to the Pmmn space group) for the positive electrode active material, the discharge potential is low at 3 V, and there is a wide range (regenerative output characteristics) where large current charging is possible.
However, along with the discharge capacity being low in the nonaqueous electrolyte secondary batteries described above, the discharge capacity decreases as the charging cycle becomes longer.
Furthermore, lithium manganese oxide with a spinel structure is used for a positive electrode active material, but the drop in discharge capacity is remarkable when the battery is kept at high temperatures.
Therefore, controlling the reduction in discharge capacity when high temperatures are maintained by using a lithium-nickel-cobalt composite oxide with a hexagonal crystal system (belonging to the R3m space group) and lithium manganese oxide with a spinel structure for the positive electrode active material has been proposed (for example, Japanese Patent Laid-open Publication No. 2000-77071).